How to calculate the value of the mains fuse?

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syn

Well-known member
Joined
Oct 11, 2005
Messages
653
Hi

How to calculate the value of the mains fuse? Do I just use 550mA fuse
for a 550mA rated transformer?

Thank you very much

m
 
You need to know what current or power your device draws.

Does it say on the back?

If not, then it sounds like a 0.5A fuse might be ok....
 
rodabod thanks

i need to know that for the various DIY equipment that i have build or i'm building at the moment? All i know is mains transformer data...

m
 
Well, the mains transformer data only tells you the maximum current (or power) that the transformer can provide.

It is possible that your circuit is drawing a lot less than what your transformer is able to provide....

You could calculate using maths what maximum current your circuit is likely to draw, or you could try and run it at max (whatever that involves) and measure using a multimeter.

Are these circuits you are using standard designs? Some people here may know what fuses to use....

Roddy
 
rodabod thanks again

shall i go to data sheets, add up max power consumption for active components, and that would be it?any hidden points? sorry if i'm p.i.a. but i also racked some of the less common ch. strips and will continue with that practice so don't want to bother community every few weeks with "what is the fuse for this whats the fuse for that".
or it is not simple like i written above?
cheers
m
 
Yes, you could add up the max current drawn for the active components...

The only hidden point might be the current used by the active components while driving signals - ie. more current will be drawn when the circuit is working harder, but this depends on the circuit and also might not make a huge difference.

There are general guidelines for chips like the NE5534 - eg. just count how many you are running and multiply by a certain amount, but I don't know if this will apply here...

Remember that your mains fuse is current at 240V (or 120V... whatever you are using in Belgrade!) and therefore the current at mains Voltage is less than the current at say, 15V.

Calculate the Power drawn in the circuit, and then divide by your mains Voltage to get the Current.

Someone else with more experience in this field can maybe chip-in here...
 
Thank you very much
i have a good starting point now.BTW we run
220V over here and all I do are mic pre\line pre\ eq stuff.
:guinness: :sam: to you
m
 
Someone over here (I think it was CJ..) said the following about this:
Short the secondaries of your power trannie and meassure the current draw (don't take too much time for this :wink: ). Choose a fuse with a lower A rating than the current draw you just meassured....
 
Thanks radiance
but i think i'll measure the transformers max A draw this
way, rather than the actual circuit max A draw.However i might be wrong saying that.
cheers
m
 
I don't know if this is the "right" way to do it, but what I do usually is to measure the current drawn by the device under normal operation. Then I use a fuse rated for about twice that value.
 
NYDave
thanks a lot, if it works for you, than i guess it is the right way.
You double A rating for the fuse in order to allow for "extra" current that a circuit might attempt to draw?
greets
m
 
The advice of radiance is good, supplemented by that from NYD. Be sure to use a time-delay ("slo-blo") fuse to handle the inrush current. Otherwise the fuse will fatigue ("work-harden") and fail prematurely.

Safety agencies test to make sure a fault condition upstream will not cause a fire. If the transformer has an internal thermal fuse then you will be protected by that. But it doesn't hurt to have an additional level of protection.
 
Recomended reading for everyone....

http://sound.westhost.com/power-supplies.htm

You can scroll down to the "fusing and protection" part but i suggest you read the whole article.
 
Seems like there are a lot of ways to select fuses. Here is a simple rule of thumb I use.

Add all the secondary VA ratings up. Divide the total by your mains voltage to get current draw. Multiply by 1.25 and use the next largest slow-blow fuse.

So, if I had a transformer with two 6VA secondaries, I would have a total 12VA. Since I have 120V system at my house, I divide 12VA by 120V and get 0.1A. Multiply 0.1x1.25 and I get 0.125. So I use a 1/8A slow-blow fuse.
 
Thanks a lot to you all.Now, this should be more than enough info for anyone tackling similar task.
regards
m
 
[quote author="syn"]......Now, this should be more than enough info for anyone tackling similar task........

[/quote]

Yeah, this thread should go to the Meta's...
 
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